In recent years, pixel sizes are rapidly becoming smaller, in response to demands for solid-state imaging elements that are smaller in size and include a larger number of pixels. Particularly, since there is a need to satisfy the demands for smaller camera modules and camera modules with a larger number of pixels, there are strong demands for finer pixels in the imaging elements for portable telephone devices with camera functions.
However, as pixels become finer, the tendency of the S/N ratio to be lower becomes inevitable. The noises generated in the photodiodes in the pixels can be divided into the components that are generated in proportion to their volumes, and the components that are generated in the interface between each photodiode and its peripheral structures. However, it is well known that the latter is the dominant components. Also, it is known that, of the components generated in the interface between each photodiode and its peripheral structures, the pixel separation face components or the components that are proportional to the product of the peripheral length of the photodiode and the depth of the photodiode observed when the photodiode is cut along a plane perpendicular to incident light are the dominant components. Only the noises in the pixel separation face components are proportional to the pixel pitch, and the other noises and signal amounts are proportional to the square of the pixel pitch. Accordingly, when only the inside of each photodiode is taken into consideration, the noises in the pixel separation face components relatively increase, and the S/N ratio becomes lower, as the pixels become finer.
In reality, most of the noise components outside the photodiodes are not affected directly by the reduction in size of pixels. However, there are noise components that increase as the pixels become finer, such as the noises in analog transistors in the pixels. Even if the noises in the pixel separation face components can be ignored, and the S/N ratio inside the photodiodes does not depend on the pixel pitch, the S/N ratio becomes lower as the pixels become finer.
To reduce the pixel separation face component noises that become dominant at the time of miniaturization of pixels, the separation structure of each photodiode or a so-called pixel separation structure has an impurity separation structure unique to solid-state imaging elements, and differs from a device separation structure of a general miniaturized LSI.
A device separation structure of a miniaturized LSI is normally a trench separation structure that has an oxide film buried in a trench structure. When this trench separation structure is applied to pixel separations in a solid-state imaging element, the dark current generated from the interface state between Si and SiO2 existing in the sidewalls of the trench separation structure generates large noises, and causes the S/N ratio to rapidly deteriorate. It is known that, to avoid the influence of the interface state between Si and SiO2, a high-density p+-type impurity diffused region is effectively formed in the sidewalls of the trench.
However, it is not necessarily easy to form such a high-density p+-impurity region. Furthermore, a trench separation and the formation of a high-density p+-type impurity diffused region around a trench lead to an increase in pixel serration area occupancy, and hinder miniaturization of pixels.
Also, it is known that there is a trade-off relationship between the width and depth of each pixel separation structure. Therefore, in fine pixels each having an impurity separation structure, there is the problem of a decrease in the volume of each photodiode due to a relative increase in pixel separation width, or a decrease in the depth of each pixel separation structure and a decrease in the depth of each photodiode.
When the depths of pixel separations and photodiodes become smaller, the amount of long-wavelength light that can be absorbed becomes smaller, and the signal amount also becomes smaller. The charges generated due to light absorption in regions deeper than the pixel separations and the photodiodes enter adjacent pixels, and so-called electrical crosstalk occurs. Although it is apparent that the former causes the S/N ratio to become lower, the latter not only lowers the spatial resolution but also lowers the S/N ratio.
Electrical crosstalk also occurs for some reasons other than the depths of pixel separations and photodiodes. Electrical crosstalk occurs in a case where the signal charges generated inside a photodiode enter adjacent pixels via a pixel separation structure. To form a high-density p+-impurity region that is a general pixel separation structure among solid-state imaging elements, it is necessary to perform low-acceleration to high-acceleration ion implantation several times. However, in a case where the depth of the bottom of the profile of injected and activated impurities or the height of the potential barrier in the portion having a low impurity density in the depth direction is not sufficiently large, electrical crosstalk might occur through that portion. To prevent the crosstalk, it is necessary to perform ion implantation many times at various acceleration voltages, and simplification of the manufacturing procedures is hindered. The electrical crosstalk through the pixel separation is formed with the components that are proportional to the pixel pitch, and the ratio of electrical crosstalk to the signal components that are proportional to the square of the pixel pitch tends to become higher as the pixels become finer.
Furthermore, it is important to restrain not only the electrical crosstalk but also optical crosstalk. In the peripheral portions of the imaging region, incident light enters the substrate not in a vertical direction but in an oblique direction. Particularly, in a solid-state imaging element for portable telephone devices with camera functions that have small-sized, low-height camera modules, the maximum incidence angle is large. The light that enters in an oblique direction cuts across a photodiode cross section obliquely. As a result, the components that are not sufficiently absorbed inside the photodiode pass through the photodiode, and reach the pixel separation structure. The main component of a high-density p+-impurity region that is a standard pixel separation structure among solid-state imaging elements is single-crystal silicon that is a semiconductor substrate material. When optically observed, there is not a boundary with the photodiode region, and the transmitted light that has reached the pixel separation region travels straight ahead. Further, the components that are not absorbed in the pixel separation region pass through the pixel separation region, and reach an adjacent photodiode. The components are then absorbed in the adjacent photodiode, and optical crosstalk occurs.
To restrain occurrences of optical crosstalk, a separation with a trench in which a material to reflect incident light is buried is effectively performed.
When the pixel separation with a trench structure is applied to a solid-state imaging element, it is necessary to form an additional high-density p+-type impurity diffused region for restraining generation of dark current, and as a result, the photodiode aperture size is restricted.
In a structure that does not require a high-density p+-type impurity diffused region, a conductive material buried in a trench structure is biased at a negative voltage, to form a hole storage layer in the vicinity of the boundary face between a photodiode and a pixel separation structure. In this manner, noises of pixel separation face components can be reduced.
However, when a negative voltage is applied to a conductive material buried in a trench structure, an electric field is always formed in the insulating material formed between the conductive material and the photodiode. If there is a defect in the insulating material, a local electric field concentration occurs, and a leakage current generated by the breakdown flows into the photodiode. As a result, defects such as white scars in an image might be caused. Furthermore, it is necessary to add a structure for applying the negative voltage.
As a technique for avoiding a decrease in the amount of incident light entering a photodiode due to a pixel-region metal interconnect layer that is another cause of a decrease in the S/N ratio in miniaturization of pixels, so-called back-illuminated solid-state imaging elements that have light entering from the back face side of a semiconductor substrate, which is the opposite side from the metal interconnect layer, are being actively developed.
After incident light passes through the microlenses and color filter array of such a back-illuminated solid-state imaging element, the light does not pass through the metal interconnect layer but directly enters the photodiodes. Accordingly, high light use efficiency is achieved, and the sensitivity becomes higher.
Also, as the distance that incident light travels to reach a silicon substrate is shortened, obliquely incident light is refracted when entering the silicon substrate having a high refractive index. Accordingly, this structure is also effective against optical crosstalk.
However, where the pixel pitch is reduced to approximately 1 μm, the depth of each photodiode is approximately 3 μm. Therefore, obliquely incident light reaches adjacent pixels, and optical crosstalk still occurs.
To achieve sufficiently high red sensitivity, the depth of each photodiode should preferably be 5 μm or greater. With such a depth, however, the above-mentioned optical crosstalk increases, or it is difficult to form a high-density p+-type impurity diffusion region at a deep location. For such reasons, the thickness of the silicon substrate in each back-illuminated solid-state imaging element is approximately 3 μM, and sufficiently high red sensitivity is not achieved.
As described above, to restrain a decrease in the S/N ratio (sensitivity) in the course of miniaturization of pixels, it is essential to reduce the noise components generated in the vicinity of each photodiode, and reduce optical crosstalk and electrical crosstalk.
To do so, pixel separation structures that can be formed in a very small size and can be formed at sufficiently deep locations are needed, so as not to compress the volumes of photodiodes. Particularly, to reduce optical crosstalk, trench-type pixel separation structures are desired.
With trench-type pixel separation structures, however, the structures necessary for reducing noises of pixel separation components compress the volumes of photodiodes, or cause white scars, or the like.